A loop heat pipe illustrated in FIG. 1A is known as a device for cooling a heat source (see, for example, Patent Documents 1 and 2 listed below). The loop heat pipe is a cooling system in which an evaporator 110 and a condenser 130 are connected in a loop via a liquid line 112 and a vapor line 113 to circulate a working fluid. As illustrated in FIG. 1B, the evaporator 110 is brought into contact with a heat source such as a CPU or the like to vaporize the working liquid 105 in the evaporator 110 making use of heat absorbed from the heat source 120. On this principle, the heat source 120 is cooled. The vapor 103 generated in the evaporator 110 is fed through the vapor line 113 to a condenser 130, at which the vapor 103 is liquefied. The liquid-state working fluid is stored in a reservoir tank 125 and supplied back to the evaporator 110.
Since the heat source 120 such as an electronic component exemplified by an LSI package is typically shaped in a flat plate, it is preferable for the evaporator 110 which serves as a heat absorber to be shaped in a flat plate so as to be brought into close contact with the heat source 120. To improve the cooling ability of the loop heat pipe, increasing the internal volume of the evaporator 110 is effective. However, there is demand for the evaporator 110 to be made as compact as possible in view of the technical trend of reduction in the size and the weight of electronic equipment. To satisfy the contradicting requirements, a flat plate evaporator is desirable because it has a compact external shape and a large internal volume.
A wick 115 made of a porous material is provided inside the evaporator case 111 so as to be in close thermal contact with the inner wall of the evaporator case 111. The working liquid 105 is driven by a capillary force of the wick 115. To efficiently vaporize the working liquid 105 penetrating through the wick 115, it has been proposed to arrange multiple wicks 115 in parallel with each other inside the evaporator case 111. This arrangement can increase the contact area between the wicks 115 and the evaporator case 111 (see, for example, Patent Document 3 listed below).
However, if heat is transferred too quickly from the heat source 120 to the working liquid 105 flowing into the evaporator 110, the working liquid 105 comes to a boil before it reaches the wick 115. As a result, bubbles 101 are generated as illustrated in a circle of FIG. 1B. Particularly, for a thin and compact flat plate evaporator, bubbles 101 are easily generated because the heat source 120 is positioned very close to the working liquid 105. The bubbles 101 prevent the working liquid from flowing into the evaporator 110, and they impede the capillary force of the wick 115, as illustrated in FIG. 1C. Without the vapor bubbles 101 in the working liquid 105, the capillary force acts toward the vapor side in the porous wick 115 (as indicated by the arrow 116) and the working liquid 105 is appropriately transported to the vapor side. In contrast, if the bubbles 101 are generated in the working liquid 105, surface tensions on the vapor side and the liquid side of the wick 115 counteract each other as indicated by the upward arrows and the downward arrows, and a sufficient capillary force is not exerted. This causes circulation of the working liquid 105 to be attenuated or prevented, and the cooling performance of the loop heat pipe is ultimately degraded.